In the prior art, equipment used to make fine beams generally consists of a needle-type source of particles which may emit electrons or cause ionization of surrounding gas, involve surface ionization at low temperatures, or most commonly is a needle wet with a liquid metal or alloy containing a desired ion. In some cases a single lens is used to form an image of the very small emitting region of the source. Greater intensity is obtained by using a condensing lens near the source or a plurality of condensing lenses. When several lenses are used, it is possible to make an intermediate image of the source; and if the intermediate image is placed at the center of an ion-optical component such as an accelerating gap or a Wien filter, the aberrations caused by the component are greatly reduced. However the focusing ability of all systems utilizing electrostatic lenses is limited by the chromatic aberration of these lenses in combination with the unavoidable energy spread of ions emerging from the ion source. The full width d of the final focal spot is given by a formula of the type EQU d=C a dE/E
where dE is the spread of energies about the central energy E of the ions, a is the full angle of convergence at the final focus, and C is a chromatic aberration coefficient. Chromatic aberration differs from other aberrations in its first-order dependence upon a. For example the spherical aberration of a round lens varies as a.sup.3. Thus 1st-order chromatic aberration is important at small lens apertures, where spherical has become very small, and 3rd-order spherical aberration will dominate at some large value of a.
The insufficient focusing ability of electrostatic systems causes limitations to ion-optical devices which have been longstanding difficulties in the prior art. In applications such as micromachining, the current density is of prime importance. The current from a liquid metal ion source is given by EQU I=B a.sub.s b.sub.s r.sup.2
where typical numbers are brightness B=10.sup.6 A/sr-cm.sup.2, emission angles a.sub.s, b.sub.s =400 milliradians, and effective source radius smaller than r=100 Angstroms. Because the effective source is so small, the size of the beam is given by d rather than by the geometrical image of the source. Accordingly the current density J is given by EQU J=I/d.sup.2 =B(ab/ab)(rE/C dE).sup.2.
Since the quantity E/C is roughly constant in electrostatic lenses it has been stated that "the maximum current density obtained of about 1 A/cm.sup.2 will not be increased substantially in the near future." Even complicated electrostatic lenses containing four electrodes produce a maximum of 10 A/cm.sup.2. To overcome this longstanding difficulty, an object of the present invention is to increase current density, by utilizing achromatic lenses in which C=0.
When covering a large specimen area is desired, such as in maskless ion implantion for fabrication of application-specific integrated circuits, the current rather than the current density becomes important. At 1 nA current, the time to write a 4-inch wafer at a dose of 10.sup.13 ions/cm.sup.2 in a single-lens system is about an hour. The low current has been a difficulty in such equipment. Larger currents may be obtained in a system with a plurality of lenses, which allow operation with larger values of a and b. However such increased angles introduce aberrations which may make the focused beam larger than the desired feature size. For angles smaller than the knee angle at which chromatic and other aberrations are equal, removal of chromatic aberration will result in a larger operating angle and a larger current for any given focused beam size. Accordingly, an object of the present invention is to produce higher currents and allow microfabrication at higher writing speeds, by utilizing complete achromatic systems in which C=0.